Endothelial dysfunction decreases the ability of the endothelium to maintain normal hydration, which may lead to corneal decompensation. Corneal endothelial function is known to be affected by some ophthalmic procedures (eg., cataract surgery, intraocular lens implants, refractive keratoplasty) and in endothelial dystrophy. Clinical assessment of endothelial function would provide a means of identifying individuals at risk from certain types of ocular therapy, allow a more definitive diagnosis of early endothelial dystrophy, and provide a quantitative method for monitoring progression of the disease. A corneal physiological stress test, designed to measure the corneal swelling/deswelling response to hypoxia, will provide an Endothelial Function Index (EFI), which will be validated for use in assessing endothelial integrity and function. The EFI will be determined for normal subjects and subjects with various degrees of endothelial dystrophy to provide criteria for identifying endothelial dysfunction. The EFI for each group will be compared to measurements of endothelial cell morphology, central and peripheral corneal thickness in order to provide important data on the relationship between the structure and function of both the normal and abnormal corneal endothelium. The long-range objective is to provide an in-office test of endothelial function that can be used by (1) the ophthalmologist to identify, before intervention, those individuals at risk of corneal decompensation, (2) the physiologist as a method to measure endothelial function, and (3) the clinician to monitor changes in endothelial function that may accompany a disease process or clinical intervention.